This invention relates to ink compositions.
Overhead projectors are widely used, particularly in academic and business settings. Overhead projectors enable a marking on a transparent film to be projected on a screen or wall by shining light through the transparent film. A speaker giving a presentation can write on the transparent film, and/or make additions or corrections to a preprinted image, while the film is on the overhead projector. This allows the speaker a flexibility which would not be possible if transparencies or other audiovisual aids were used.
Conventional inks for use on overhead projector (OHP) film fall into two categories: permanent OHP inks and erasable OHP inks. Most erasable OHP inks are xe2x80x9cwashable,xe2x80x9d i.e., water is required to remove a marking made with the ink. This can make removal of the markings messy and inconvenient, and also can make it difficult to erase a small portion of a marking.
The invention features aqueous inks for use in marking overhead projectors. These inks form markings of excellent quality and light transmission on, and are readily removable from most commercially available overhead projector films. The inks are dry-wipeable, i.e., a marking made on an overhead projector film with the ink may be removed, after drying, by wiping the marking with a dry cloth (i.e., a cloth, felt eraser, tissue, or the like). Advantageously, by dry wiping, a small area of a marking can be easily removed, without interfering with the rest of the marking. Markings made with preferred compositions may also be removed by the user by wiping the marking with a fingertip. Further, the markings formed with the inks wipe off cleanly, with minimal dust formation, and resist inadvertent removal, e.g., when sheets of OHP film are shuffled together or photocopied. Some preferred compositions are also water wipeable, as well as dry-wipeable.
The inks of the invention include a colorant, a polymer having a film-forming temperature below room temperature, and water.
In preferred compositions, the polymer is an acrylic polymer, preferably provided in the form of an emulsion. The acrylic polymer preferably has a glass transition temperature (Tg) of from about +20xc2x0 C. to xe2x88x9240xc2x0 C. and a film forming temperature below room temperature (25xc2x0 C.), preferably below 20xc2x0 C. Preferably, the ink further includes a surfactant, more preferably a silicone surfactant, and an anti-drying agent, more preferably a humectant, most preferably glycerol.
The term xe2x80x9cfilm forming temperature,xe2x80x9d as used herein, refers to the temperature at which the film forming polymer forms a continuous dry film. The term xe2x80x9cglass transition temperature,xe2x80x9d as used herein, refers to the temperature at which the mobility of randomly cited long chain molecules in the film forming polymer is lost.
It has been found that, by adjusting the pH of the ink composition, the ink can be rendered washable as well as dry-wipeable. Accordingly, if washability is desired, the compositions further include a base, e.g., a buffer. Preferably, sufficient base is added to adjust the pH of the ink composition to from about 7 to 10.
Preferred compositions may also include a peeling agent, e.g., a latex polymer, to enhance removability of the dry marking, and/or a fluoropolymer, to extend the period of time over which the marking remains dry-wipeable.
Other features and advantages of the invention will be apparent from the Description of the Preferred Embodiments and from the claims.
Preferred marker inks include a surfactant, an acrylic film-forming polymer, an anti-drying agent, a pigment, and water.
The surfactant is preferably a silicone surfactant. Suitable silicone surfactants include, but are not limited to, silanes and siloxanes. Preferred silicone surfactants are polyalkylene oxide-modified methylpolysiloxanes, e.g., those commercially available from Union Carbide under the tradename SILWET. Preferred compositions contain from about 0.1 to 5 weight percent surfactant, more preferably from about 0.5 to 2 weight percent. Lower levels may reduce wetting of the OHP film by the ink, while higher levels may cause too much adhesion of the ink to the film.
The acrylic polymer is preferably provided in the form of an emulsion. Typical emulsions have a solids level of about 50%. If desired, a water soluble acrylic may be used and predissolved in the water which will be contained in the ink composition. Preferred acrylic polymers have a glass transition temperature of from about +20xc2x0 C. to xe2x88x9240xc2x0 C. and a film forming temperature below room temperature, more preferably below 10xc2x0 C. Preferred acrylic polymers include those commercially available under the tradename G-CRYL from Henkel Corp., and the tradename ACRYSOL from Rohm and Haas. Particularly preferred acrylic emulsions are G-CRYL 1200 and ACRYSOL I-1955. Acrysol I-1955 has a glass transition temperature of 26xc2x0 C. Preferred compositions contain from about 5 to 15 weight percent of the acrylic polymer. Too much of the acrylic may result in undesirably high viscosity at an effective level of resin and may reduce the removability of the dried marking, while too little may result in an ink which does not form a coherent film on all overhead projector films.
The anti-drying agent is preferably a humectant, more preferably glycerol. Glycols are typically less preferred, as they may cause smearing when the marking is dry-wiped. The anti-drying agent prevents dry-out of a marker containing the ink when its cap is left off, and also significantly enhances removability of the dried marking. Too much of the anti-drying agent may cause the ink to dry too slowly, while too little may not prevent drying out of the marker and may reduce removability. Preferred compositions include from 1 to 25 weight percent of the humectant, preferably from about 5 to 20 weight percent.
Enough pigment should be included in the ink to provide an adequate color intensity, but not so much that light transmission is compromised. The preferred inks include between approximately 2% and 20% pigment by weight, more preferably between approximately 5% and 10%. Pigments that may be used in the invention include water dispersible pigments, and preferably the pigment is provided in the form of an aqueous dispersion, for accurate measurement and uniform mixing. Preferred pigment dispersions contain an acrylic dispersant, as these dispersions are most compatible with the acrylic polymer component of the composition. Suitable pigment dispersions are available from HeucoTech, under the tradename HEUCOSPERSE III. It is preferred that the pigment have a relatively small particle size, preferably less than 0.5 micron, to prevent settling and for use in capillary feed markers.
Sufficient water should be included in the ink so that its Brookfield viscosity at 25xc2x0 C. is less than about 20 cps, more preferably between about 2 cps and 10 cps. If the viscosity is too low, the composition may not contain effective amounts of the active ingredients. If the viscosity is too high, the ink may be too thick for practical use, particularly in capillary feed systems.
Preferred compositions may, optionally, include a small amount of a peeling agent, to enhance the removability of the dried marking. Preferred peeling agents are latex polymers, more preferably styrene butadiene rubber (SBR) latexes. A suitable SBR latex is available from Goodyear under the tradename PLIOLITE. It is preferred that the composition contain from about 0 to 5 weight percent of the SBR latex. Too much of the peeling agent may cause a marking made with the ink to be crumbly or smeary.
Preferred compositions may also include a base, e.g., a buffer, if it is desired that markings formed by the ink be water washable as well as dry-wipeable. Acid stabilized acrylic emulsions are generally preferred for optimal washability with a wide variety of bases. A suitable emulsion is ACRYSOL I-1955 emulsion, available from Rohm and Haas. Suitable buffers include but are not limited to potassium hydroxide, sodium hydroxide, sodium carbonate and 2-amino-2-methyl-1-propanol. Preferred washable compositions contain from about 0.1 to 5 weight percent of the buffer. The preferred amount will depend upon the strength of the base selected and the initial pH of the composition. Preferably, a sufficient amount is included to bring the Ph of the ink to about 7 to 10.
Markings made with the preferred inks typically become permanent after a period of time, e.g., a week after the marking is made. This may be advantageous, if the user desires to store the marked OHP film for future use. However, if it is desired that markings made with the ink remain dry-wipeable over long periods, a fluoropolymer may be added to the ink to enhance long-term removability. A suitable fluoropolymer is available from Eastern Color and Chemical under the tradename RANE-OFF F-7. Preferred compositions contain from about 1 to 10 weight percent fluoropolymer.
Other conventional ingredients may be included in the composition, e.g., biocides.
A preferred procedure for preparing the compositions of the invention is to add the surfactant to the water, then add the acrylic emulsion, then the anti-drying agent, and finally the pigment dispersion, mixing between each addition. The order of addition is not critical however, and other procedures can be used, as is well known in the art. The ingredients can be mixed using any conventional mixer, under conditions of relatively low shear.